The present invention relates to an arc welding power source.
FIG. 1 is a diagram of an electric circuit of the construction of a conventional arc welding power source. In FIG. 1, reference numeral 1 designates 3-phase inputs. Numeral 2 designates a first rectifier circuit for converting the 3-phase inputs into a D.C. voltage; numeral 2a designates a smoothing capacitor; numeral 3 designates an inverter for converting the D.C. voltage into an interrupted A.C. voltage; numeral 4 designates an output transformer for stepping down the interrupted A.C. voltage; numeral 5 designates a second rectifier circuit for converting the stepped down interrupted A.C voltage into a D.C. voltage together with a filter reactor 6; numeral 7 designates a welding load; numeral 8 designates a current detector for detecting the current of the welding load 7; numeral 9 designates a pulse width modulator (hereinafter referred to as "PWM") circuit; and numeral 10 designates a PID current control circuit for controlling the output of the current detector 8 so as to coincide with a first current target value determined by combining an input signal with its integrated and differentiated signals. Numeral 11 designates a first target setting circuit for setting the first current target value.
In the conventional arc welding power source of the above-mentioned construction, the 3-phase inputs 1 are converted by first rectifier circuit 2 into the D.C. voltage. The converted D.C. voltage is converted by the inverter 3 into the interrupted A.C. voltage. This interrupted AC voltage is further stepped down by the output transformer 4 and then converted into a welding D.C. voltage by the second rectifier circuit 5 and the filter reactor 6. The converted welding D.C. voltage is supplied to the welding load 7. The PID current control circuit 10 is operated so that the output signal fed back from the current detector 8 for detecting the current of the welding load 7 coincides with the output signal of the first target value setting circuit 11. The output of the PID current control circuit 10 is pulse-width-modulated by the PWM circuit 9, and then transmitted to the inverter 3.
Since the PID current control circuit 10 has a slight response delay for the variation of the welding load 7, the output current largely exceeds the target value as shown in FIG. 2 when the welding load 7 abruptly varies, i.e., switches from the arc to a shortcircuit. Therefore, an adequate current waveform is not formed, the arc is regenerated momentarily from the shortcircuit due to the large current to cause the welding to become unstable such as the transfer of the welding droplet to become irregular, or when the current exceeding the target value at the shortcircuiting time is large, the inverter 3 might be damaged.